14 



NATURE 



[September 6, 191; 



OVR ASTRONOMICAL COLUMN. 

 Solar Radiation and Terrestrial Meteorology.— 

 In view of the evidence obtained by Abbot as to short- 

 period changes in the intensity of solar radiation, 

 Mr. H. Helm Clayton, of the Argentine Meteorological 

 Service, has investigated the possible coincidence of 

 these variations with atmospheric changes on the 

 earth (Smithsonian Miscellaneous Collections, vol. 

 Ixviii., No. 3). Comparison was first made with 

 temperature observations at Pilar, in Central Argen- 

 tina, and afterwards, as regards both temperature 

 and pressure, at a number of widely distributed 

 stations. The pressure correlation was found to_ be 

 the reverse of that of the temperature. In the tropical 

 regions the temperature rises and falls in unison with 

 the changes of solar radiation, but follows the solar 

 changes by about two days ; following a rise of tem- 

 perature, the pressure falls, reaching a minimum 

 between the second and third day after the solar 

 change. On the succeeding day tlie pressure attains 

 a> maximum in the temperate region and the tem- 

 perature a minimum. Four to five days after the 

 solar changes there is a minimum of pressure in the 

 Arctic Circle near the 6oth parallel in both hemi- 

 spheres, and a maximum of temperature in the 

 oceanic centres of low pressure like that near Iceland. 

 These results are interpreted as indicating a trans- 

 ference of air from the tropics to high latitudes, 

 probably in the upper layers. Analysis of the solar 

 variation suggests a period of about twenty-two days, 

 which was also shown by the fluctuations of tempera- 

 ture at Buenos Aires during the same period. Con- 

 tinued observations of solar radiation are regarded 

 as being of great importance for meteorology. 



Proper Motion of the Great Andromeda Nebula. 

 — Prof. Barnard has recently given an account of his 

 attempts to detect proper motion of the great nebula 

 in Andromeda {Astronomical Journal, vol. xxx., 

 No. 20). The nucleus of the nebula is about 2" to 3" 

 in diameter, but it is so strongly condensed that under 

 good conditions it can be bisected with almost the 

 same accuracy as tl-ie comparison stars. In 1898, in 

 the hope of ultimately detecting motion of the nebula. 

 Prof. Barnard began a series of measurements with 

 respect to three small stars which seemed to have no 

 connection with the nebula. The observations' were 

 repeated in 1909, and again in 1915-16, but notwith- 

 standing the lapse of eighteen years, no displacement 

 could be detected. Previous measurements by other 

 observers are somewhat discordant, but appear to show 

 that no considerable motion has occurred during the 

 past eighty years. The individual measures by Prof. 

 Barnard show that the parallax must be beyond the 

 reach of ordinary micrometer work. In the course of 

 these observations the place of the nova of 1885 was 

 carefully examined, but nothing was seen in this posi- 

 tion. 



The Lunar Eclipse of July 4. — During the totaJ 

 phase of the eclipse of the moon on July 4-5, 1917, 

 it was remarked by several observers that the bright- 

 ness of the disc was sensibly greater near the limb 

 than towards the centre. It has been suggested by 

 M. a. Nodon that this appearance may possibly indi- 

 cate a feeble luminosity of the surface of the moon 

 {L'Astronomie, August). An experiment which ap- 

 pears to support this view is described by M. Nodon. 

 A brass ball about 10 cm. in diameter was placed in a 

 dark box, of which only one side was oper^, and was 

 viewed in a feeble light; the appearance observed was 

 that of a disc brightest at the centre. On the other 

 hand, in the case of a sphere which was uniformly 

 coated with a slightly phosphorescent substance, the 



NO. 2497, VOL. 100] 



luminosity was greater at the edges than at the centre. 

 Phosphorescence of some of the materials composing 

 the lunar surface is accordingly suggested as a possib! 

 explanation of the distribution of luminosity observe 

 during the eclipse. 



THE MODERN RANGE-FINDER. 



THE War Office has published a pamphlet on 

 the modern range-finder, written by Prof. 

 Cheshire, and, as is to be expected from an author of 

 such technical knowledge, it is a clear and thorough 

 exposition of a difficult and attractive subject When 

 it is considered that all that a range-finder has to do is 

 to enable the observer to utilise the angle of converg- 

 ence upon a distant object of the widely spaced eyes 

 of the instrument in order to find the distance of the 

 object the problem of range-finding may appear to be 

 very simple, and so in principle it is. This is not the 

 difficulty. The real difficulty is to make an instru- 

 ment which shall be portable, handy, and quick in use, 

 and also shall attain the ultimate possible limit 

 of accuracy. That which is not only attainable, but 

 attained every day is something so perfect as to exceed 

 the utmost that an inventor might have dared to hope 

 for. Some form of reflecting device is needed at each 

 end to bring the two sets of optical beams together 

 into a single eyepiece. Any structure that supports the 

 mirrors or prisms is liable to bend under its pwn weight 

 or on account of differential heating. Simple reflectors 

 at the ends would double any such angular displace- 

 ment, and the kind of accuracy required would be un- 

 attainable. Double-reflection prisms, however, may 

 be tilted without affecting the apparent direction of 

 the object, as may be noticed when using the ordinary 

 camera lucida. However, such oblique reflection 

 would require prisms of inconvenient size; accordingly 

 pentagonal prisms are used, which, however, require 

 to have their reflecting faces silvered, as they are 

 within the critical angle. As these prisms turn the 

 beam through an invariable angle, slight flexure 

 such as is here contemplated does not matter. 

 The prismatic devices near the eyepiece designed 

 to bring the two beams in two parts of the 

 field into view together and into perfect ajignment, 

 where the object is at a very great distance, must not 

 only do this, but the line of demarcation between the 

 fields should be sharp throughout its extent. This is 

 essential to accuracy. These fields may both appear 

 erect, or one may appear inverted either laterally or 

 vertically. Where there is convergence of the beams 

 the alignment is disturbed, and the optical means by 

 which it is corrected, as by a sliding prism, are con- 

 nected up with a scale, so that the distance may be 

 read directly. In the B0rr and Stroud range-finder, 

 which is more particularly described and illustrated, 

 this scale is seen by the other eye through a separate 

 eyepiece. It is satisfactory to find that in ttie essential 

 of sharpness of the line of demarcation the Barr and 

 Stroud instrument is superior to two German forms. 

 It is quite impossible in the limits of space here avail- 

 able even to indicate the nature of the highly ingenious 

 three-dimension reflecting devices which serve to bring 

 the two converging beams into sharply separated parts 

 of the field, and in the Barr and Stroud instrument at 

 the same time to throw them up at an angle of 60°, so 

 that the observer lying on the ground or in other com- 

 fortable position may look down at a convenient angle 

 instead of wearinf^ out his neck by looking horizontally. 

 In one form of instrument made by Zeiss the tele- 

 scopic magnification of the two beams is different, so 

 that the images seen in juxtaposition are of different 



